Optical scanner devices are well-known in the art and produce machine-readable image data signals that are representative of a scanned object, such as a photograph or a page of printed text. In a typical scanner application, the image data signals produced by an optical scanner may be used by a personal computer to reproduce an image of the scanned object on a suitable display device, such as a CRT or a printer.
A hand-held or portable optical scanner is an optical scanner which is designed to be moved by hand across the object or document being scanned. The hand-held scanner may be connected directly to a separate computer by a data cable. If so, the data signals produced by the hand-held scanner may be transferred to the separate computer "on the fly," i.e., as the image data are collected. Alternatively, the hand-scanner may include an on-board data storage system for storing the image data. The image data may be downloaded to a separate computer after the scanning operation is complete by any convenient means, such as via a cable or an optical infrared data link.
Hand-held or portable optical scanners are well-known in the art and various components thereof are disclosed in U.S. Pat. No. 5,552,597 of McConica for "Hand-Held Scanner having Adjustable Light Path", U.S. Pat. No. 5,586,212 of McConica, et al., for "Optical Wave Guide for Hand-Held Scanner," U.S. Pat. No. 5,381,020 of Kochis, et al., for "Hand-Held Optical Scanner with Onboard Battery Recharging Assembly," and U.S. Pat. No. 5,306,908 of McConica, et al., for "Manually Operated Hand-Held Optical Scanner with Tactile Speed Control Assembly," all of which are hereby incorporated by reference for all that they disclose.
A typical hand-held optical scanner may include illumination and optical systems to accomplish scanning of the object. The illumination system illuminates a portion of the object (commonly referred to as a "scan region"), whereas the optical system collects light reflected by the illuminated scan region and focuses a small area of the illuminated scan region (commonly referred to as a "scan line") onto the surface of a photosensitive detector positioned within the scanner. Image data representative of the entire object then may be obtained by sweeping the scan line across the entire object, usually by moving the hand-held scanner with respect to the object. By way of example, the illumination system may include a light source (e.g., a fluorescent or incandescent lamp or an array of light emitting diodes (LEDs)). The optical system may include a lens and/or mirror assembly to focus the image of the illuminated scan line onto the surface of the detector. Alternatively, a "contact image sensor" (CIS) may be used to collect and focus light from the illuminated scan region onto the detector.
The photosensitive detector used to detect the image light focused thereon by the optical system may be a charge-coupled device (CCD), although other devices may be used. A typical CCD may comprise an array of individual cells or "pixels," each of which collects or builds-up an electrical charge in response to exposure to light. Since the quantity of the accumulated electrical charge in any given cell or pixel is related to the intensity and duration of the light exposure, a CCD may be used to detect light and dark spots of an image focused thereon.
The term "image light" as used herein refers to the light that is focused onto the surface of the detector array by the optical system. Depending on the type of scanner and the type of document, the image light may be reflected from the document or object being scanned or it may be transmitted through the object or document. The image light may be converted into digital signals in essentially three steps. First, each pixel in the CCD detector converts the light it receives into an electric charge. Second, the charges from the pixels are converted into analog voltages by an analog amplifier. Finally, the analog voltages are digitized by an analog-to-digital (A/D) converter. The digital signals then may be processed and/or stored as desired.
While portable image scanners of the type described above are being used, they are not without their problems. For example, the quality of the image data produced by a scanner is usually related to the quality of the illumination that is provided to the scan region. If the illumination is not sufficiently bright, the result may be excessive noise in the image data which may manifest itself as "snow." Other problems may appear if the level of illumination is not substantially uniform along the length of the scan line. In such cases, the resulting image data may be of variable quality along the length of the scan line. Extreme variations of illumination may even result in "drop outs" in the image data.
Still other problems may develop if the illumination system is to be used with a battery powered hand-held or portable image scanner. For example, in order to minimize the power drain on the battery most portable image scanners are designed to provide the minimum amount of illumination commensurate with good image quality. Unfortunately, however, such low power illumination systems provide little margin against illumination variations. Consequently, even slight variations in illumination may well result in portions of the scan line being insufficiently illuminated. Another problem with many illumination systems is that only a small fraction of the light produced by the light source is used to illuminate the scan region.